Increasingly, user demands on telephone facilities have far exceeded the basic requirement for a clear voice connection between calling and called parties. In particular, the business community, knowing of special business features such call forwarding, conference calling, and speed dialing, have placed heavy demands both on telephone companies and equipment suppliers to provide apparatus and service that will economically satisfy such demands.
Moreoever, it is expected that these business features will be available through a central data and voice communications facility, either as a conventional telephone office switching center or as a privately owned counterpart in the form of a private branch exchange (PBX) or a private automatic branch exchange (PABX).
Previously, additional telephone features of the type indicated generally required the use of additional wires in the form of a cable. Economy suffered as a result in view of higher cable costs in both initial procurement and installation. In this regard, many private telephone systems require a minimum of four wires connecting subscriber sets to the telephone system that serves them. Since the cables of such systems are infrequently used throughout the course of a business day, many companies are reluctant to commit substantial expenditures for such installations, especially since a second pair of wires could be used for other purposes such as data transmission. Moreover, four-wire transmission is incompatible with existing two-wire transmission in the outside plant. Furthermore, power requirements of such multi-wire systems are by no means insubstantial which generally limits the range of a subscriber set in a PBX to within, at most, one thousand metres of the switching center. Accordingly, a company occupying a large building or having staff members in several outlying buildings would likely experience difficulty in obtaining business features at all locations.
A further problem that may be experienced as a result of the one thousand metre limitation relates to a company that may be too small to justify purchasing its own PBX and is either unwilling or unable to share jointly in ownership of a PBX with other small companies in the same building. Thus, a small company is disadvantaged in terms of its ability to obtain business services.
Recognition of these problems has resulted in the development of apparatus and systems that are especially adapted to share a common signalling path for voice signals and digital message signals together with auxiliary digital signals. Two typical prior art patents describing the use of a single pair of wires carrying a plurality of signals as noted may be seen in U.S. Pat. No. 4,171,467 issued Oct. 16, 1979 to L. N. Evenchik and U.S. Pat. No. 4,178,480 issued Dec. 11, 1979 to R. L. Carbrey. Both patents relate to signal multiplexing and disclose a signal multiplexing circuit that makes use of a single pair of wires to interface a data and voice communication circuit such as an electronic telephone (EKT) station set with a central data and voice communication system such as a business communication system (BCS).
To the extent indicated in the Evenchik and Carbrey patents, the problems associated with providing a telephone system having business features are substantially overcome. One problem remains outstanding, however, and is not fully addressed by either Evenchik or Carbrey.
This is the relatively inefficient use of a central data and voice communication facility to reduce user problems as, typically, when noise on the line creates a transmission error and, perhaps more commonly, when requiring the facility to resolve caller priorities should two EKT station sets attempt transmission at the same time. Under these circumstances some portion of the facility is occupied for a discrete interval and, in terms of total numbers of such occurrences over an extended period of time, an effective reduction of facility capacity results.